1. Field of the Invention
The present invention relates to an image forming apparatus such as electrophotographic copying machines, printers, or facsimiles, and an image forming method.
2. Description of the Related Art
Image forming apparatuses such as electrophotographic copying machines or laser beam printers perform what is called process control, in which control to adjust the density of images is conducted in order to provide stable image quality, for example, when the main power is turned on, during standby after a predetermined period of time has elapsed, or during standby after a predetermined or greater number of sheets have been printed. As a technique for making adjustment of an image density, for example, a technique disclosed in Japanese Patent Application Laid-open No. 2006-113540 is known.
The technique disclosed in Japanese Patent Application Laid-open No. 2006-113540 is conducted in the following steps to make adjustment of an image density. That is, first, the process develops a plurality of toner patches while varying the potential of development (the difference between developing bias and electric pattern potential), thus forming a gradation pattern of a plurality of toner patches at different densities on an image carrier such as an intermediate transfer belt. Then, the process causes an optical sensor to detect each toner patch of the gradation pattern formed on the image carrier, and calculates the amount of deposited toner of each toner patch using a predetermined algorithm on the basis of the value detected by the optical sensor. Then, on the basis of the relationship between the amount of deposited toner of each toner patch and the development potential at which each toner patch was formed, the process determines straight line equation y=ax+b. Thus, the process finds a development γ (the slope “a” with the horizontal axis representing the development potential and the vertical axis representing the amount of deposited toner) and a development start voltage Vk (the intercept “b” with the horizontal axis representing the development potential and the vertical axis representing the amount of deposited toner). On the basis of the development γ and the development start voltage Vk thus determined, the process adjusts image forming conditions such as the LD power, electrostatic charge bias, and developing bias, thereby allowing the development potential to provide an appropriate amount of deposited toner (i.e., image density).
However, according to the image density adjustment control disclosed in Japanese Patent Application Laid-open No. 2006-113540, the process continually develops a plurality of toner patches and therefore a toner patch is affected by the history of the just previously formed toner patch and made unstable when there occurs an effect of an image history at the pitch of the developing roller of the developing unit. This would lead to a problem that the process cannot accurately sense the development γ and the development start voltage Vk, resulting in the controlled amount of deposited toner (image density) being made unstable. As used herein, the term “image history” refers to the phenomenon that, by the just previously formed image, next image is affected to thereby experience variations in density.
As a technique intended to reduce such an effect of the image history to properly make adjustment of an image density, for example, a technique disclosed in Japanese Pat. No. 3,719,372 is known. According to the technique disclosed in Japanese Patent No. 3719372, a plurality of toner patches are formed while the developing bias is being varied across the variable range. After that, the process forms a defective toner removal image and then the next image that has a width equal to or greater than the width of the toner patch in the main-scanning direction and a length equal to or greater than the circumferential length of the developing roller in the sub-scanning direction, before forming the next image. The technique disclosed in Japanese Patent No. 3719372 forms the defective toner removal image before forming a new image in this way in order to equalize the amount of electrostatic charge of the toner supplied from the developing roller and prevent the next image from being destabilized due to the effect of the image history.
Japanese Patent Application Laid-open No. 2006-47841 describes a configuration forming which a toner patch detectable by a specular reflection light-receiving section and a toner patch detectable by a diffuse reflection light-receiving section are formed and detected by an integrated specular and diffuse reflection sensor to sense the toner patches while switching between emission currents flowing through light emitters. Furthermore, the technique disclosed in Japanese Patent Application Laid-open No. 2006-47841 is configured to provide a gap between both the toner patches in order to detect one patch and control the current flowing through the light-emitting diode (LED) to thereby stabilize it and then detect the other patch.
However, the technique disclosed in Japanese Patent No. 3719372 is configured to form the defective toner removal image in order to reduce the effect of the image history. This causes a waste of toner in forming the defective toner removal image, leading to an increase in running costs.
Furthermore, the technique disclosed in Japanese Patent Application Laid-open No. 2006-47841 sets the intervals between patches without taking the effect of the image history into account. Japanese Patent Application Laid-open No. 2006-47841 does not describe specific intervals between patches, either.